CMMG’s 9mm Barrel and BCG Kit allow you to build a Pistol Caliber Carbine using CMMG’s Patent Pending Radial Delayed Blowback System. Unlike typical direct blowback systems, RDB uses a bolt that is forced to rotate and unlock, slowing the BCG. This results in a lighter BCG for a lighter rifle, and less felt recoil than a straight blowback system. Their Barrel and Bolt Kits are designed to work exclusively with mil-spec AR-15 upper receivers with a modified ejection port, like their MkG/MkGs upper, and most mil-spec AR15 lowers when used with 9 AR Conversion mags as well as their MkGs Mk17 and Mk9* lowers. Other manufacturers receivers may work, but we cannot guarantee function or reliability.*Older Mk9 lowers will require one of their retrofit kits to convert it to a Radial Delayed Blowback compatible lower, instead of Traditional Blowback.